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No matter what gear you are in the best engine efficiency will be at about 2 inches of vacuum. (1.5 is fine as long as you do not floor the accelerator)
Adjust your rate of acceleration using the gears while maintaining 2 inches of vacuum.
When at a constant speed use the highest gear practical and try to keep the vacuum as low as practical.
Think of it this way. If you want the most power for the least fuel 2 inches of vacuum is a good point to get it. Your engines best fuel consumption for each horsepower produced is probably in the 1500-3000 RPM range. 4000 is probably too high.
The vacuum reading you see on the gauge is the percentage of atmospheric pressure that is going into the cylinders. If it is half of atmospheric pressure (30 atmospheric 15 reading on the gauge) then you are only achieving half of the engines designed compression ratio, or about what a model T Ford got a full throttle.
While this may seem to be counter intuitive, let me give you and example.
A 4 cylinder engine on a dyno producing 20 HP at 1500 RPM uses 1 unit of fuel. Increase the load on that same engine at the same speed to 50 HP and you only use .5 more units of fuel.
The first 20 HP cost you 1 unit. The second 30 of 50 total cost you half as much fuel as the first 20. This uses more fuel per unit of time but less fuel per unit of hp.
1=20 or 20 to 1
1.5=50 or 13.33 per half unit or 26.66 per unit.
That is why you should accelerate briskly to your desired speed. It cost less fuel.
Full throttle is to much. Higher RPMs are to much, and accelerating very gradually does not get you to peak efficiency. You need to find the "just right" balance.
This "balance" is going to be the rate of acceleration of normal traffic moving away from a light (your acceleration environment) that you can obtain using the highest possible gear. If your gear is to high then you cant maintain the rate.
Accomplish this with 2 inches of vacuum when you accelerate and you should get to speed with the least possible amount of fuel.
When you are travelling at a constant speed try to keep the vacuum reading as high as possible and as steady as possible. This will get you the best mileage as long as you are not using pulse a glide to improve your mileage further.
Pulse and glide employs an "all or nothing" principle. Accelerating at peak efficiency, then coasting with the engine either idling or turned off.
Lets say your engine used .25 gallons of fuel per hour idling and you are going 50 MPH in a coast. You are getting 200 MPG. With the engine off you are getting infinite MPG.
These coasting periods at astronomically high MPG can really increase your overall MPG if you have the patience and dedication to employ the tactic.
regards
Mech
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